11.8 is too close! (insert gay smiley thing with tongue out)
I have to agree that the HW pump is a Heath Robinson affair. I have found that you need to give it about ten good pumps to charge the pump and line before it starts to fill the cylinder. Without a gauge it is hit or miss to know the right amount (see one of my first posts about letting fly with a super sonic .17 pellet in the back garden!) I also found that tapping the release valve can dump the whole lot in one go too!
Certainly the stirrup pump is the way forward and I'm suprised no-one has made an adaptor for the HW90 et al before. I'm sure you'll have a few interested parties for it. Good work fella.